This invention relates generally to television signal transmission systems and methods and specifically concerns a novel television transmission system of improved noise performance. In an amplitude modulation (AM) implementation of the invention, the enhanced signal to noise ratio of the transmitted signal may be converted, in whole or in part, to a substantial reduction in transmission power without discernible degradation in signal fidelity. In its frequency modulation (FM) implementation, the reduced bandwidth of the transmitted signal enables much improved signal to noise performance. The preferred embodiment of the invention provides special benefits with regard to adjacent channel and co-channel interference problems with NTSC and other type television signal transmission and receiving systems. The improved transmission system permits operation in or adjacent to NTSC television signal transmission areas without objectionable adjacent channel and co-channel interference, either to or from the NTSC channel. In its FM implementation, the invention will find ready application in connection with Direct Broadcast Satellite (DBS) transmission systems. This use will result in receiving dish antennas of significantly smaller diameter and trigger a rapid expansion in growth of satellite television receivers.
An NTSC television signal occupies a 6 MHz bandwidth and imposes significant transmission power demands. These power demands are directly related to the cost of operating the signal transmitter and reductions therein can yield significant economic benefits. Also of great importance is the fact that cable television plants, especially those of older vintage, are restricted in the number of channels they can handle by the lo signal power handling capabilities of their amplifiers. It would be highly desirable to reduce the amount of signal power required to transmit television signals, thereby reducing transmitter operating costs and permitting a larger number of television channel signals to be handled by a cable plant of given power handling capability. The system of the invention achieves a marked reduction in the power required to transmit an AM television signal without discernible degradation of signal fidelity and therefore provides a solution to these needs of the prior art.
In its FM implementation, the system of the invention enables transmission with significantly less bandwidth since the deviation of the FM signal is minimized. The smaller bandwidth directly improves the signal to noise performance of the system, which improvement may be translated into smaller receiving antennas. Thus, the system of the invention will be seen to solve a longstanding problem in DBS transmission systems.
The inventive system has a number of important aspects. In accordance with a fundamental aspect of the invention, a television signal is configured such that the transmitted signal is a "hybrid," that is it has a coded (digital) portion, representing signal components of low picture detail and relatively high transmission power demand that may be transmitted in a relatively low power utilization format and an analog portion, representing signal components of high picture detail and relatively low transmission power demand. The demarcation between the analog and digital portions is a function in part of the availability of means to transmit the digital data. In accordance with the invention, the data is sent in non-active video portions of the transmitted signal.
The hybrid system is subdivided into a "basic hybrid," in which the removed and digitized low frequencies are under about 15 KHz and an "extended hybrid" in which the removed and digitized signal frequencies are under about 200 KHz. As will be explained in detail below, in the basic hybrid form of the invention, the digital part comprises video components below the line deflection frequency that are digitally coded and transmitted as data during non-active video portions of the television signal. In the extended hybrid form of the invention, the digital part comprises video components below about 200 KHz that are digitally coded and transmitted as data during non-active video portions of the television signal. Since the invention may be used with many different television signal formats, the non-active video portions of the signal may include either or both of the horizontal and vertical blanking intervals.
It has also been found that further benefits are obtained by sequentially applying basic hybrid processing and extended hybrid processing (referred to as two-step processing), with the basic hybrid processing being performed for the active video of each horizontal line such that the low frequency average of each horizontal line is removed from the analog signal. The remaining components below 200 KHz are subsequently removed.
Another aspect of the inventive system is the utilization of a 2.8 MHz double sideband AM suppressed carrier that is located in the center of the 6 MHz channel. This arrangement helps minimize adjacent channel interference from and to NTSC television signals and contributes to the ability to operate in the vicinity of other, normally restricted, television signals.
A further important aspect of the inventive system involves "temporal preemphasis," also referred to as temporal filtering, field processing or frame combing. With this approach, transmission power for stationary images is reduced while transmission power for moving images is increased. Since the average television picture is, relatively speaking, static, the use of temporal preemphasis is of benefit because the greater interference potential of the signal corresponding to moving images is outweighed by the fact that noise in a moving image is much less noticeable (visually) than noise in a stationary image. Temporal de-emphasis, of course, is applied in the receiver. This aspect of the invention has advantages in any "video" transmission system, independent of the hybrid processing of the signal. This is due to the fact that normally there is little change between successive frames of video and emphasizing the changes relative to the static portions results in very efficient transmission.
Yet another important aspect of the inventive system involves compressing the "hybrid" video signal to achieve a large signal to noise ratio for broad, flat video areas, where noise is readily discernible visually, and a low signal to noise ratio for narrow video components, representing edges and video detail, in which noise is much less visually discernible. In the receiver, the signal is expanded to undo the compression in the transmitter. The combination of compression and expansion is referred to as "companding".
Still another important aspect of the inventive system is the use of dispersal filtering to reduce the amplitudes of the peak video components by distributing their energy among the "voids" created in the hybrid video signal. As will be seen, these voids are the direct result of hybrid processing of the video signal whereby low frequency analog components are removed, coded and included as data in the non-active video portions of the remaining analog high frequency components. Dispersal filtering techniques for horizontally and vertically dispersing signals and for weighting the group delay characteristics of the dispersal filters to maximize the video appearance of the signal are also used.
As those skilled in the art will readily perceive, reduction of the average power of the transmitted signal is highly desirable, especially where adjacent channel and cochannel interferences are concerned. The reduction occurs because of the hybrid processing of the video signal which effectively replaces low frequency video signals with "doublets" that define the edges of the video image. With temporal filtering, the largest signals result from moving video edges which can be compressed even more. Companding increases the signal to noise performance for relatively stationary edges of video images at the expense of much less observable noise associated with moving video image edges. Dispersal filtering primarily reduces the amplitudes of the signals above the hybrid processing frequency range.
Other advantages flow from application of the various aspects of the inventive system that improve signal to noise performance, especially with respect to the ability to operate in an environment of adjacent and co-channel NTSC signals. One is the technique of precise carrier frequency offset with respect to co-channel NTSC signals to cause "break up" of interfering signals, and thereby reduce their visibility in the video display. Co-location, or locating the hybrid signal transmitter of the invention close to the adjacent channel NTSC transmitter, may be used to assure that receivers in both reception areas receive approximately equal strength signals to enable their respective AGC systems to set up properly. Frame locking the hybrid signal to the NTSC signal and incorporating all data in the vertical blanking interval of the hybrid signal contributes to the ability to operate in a co-channel environment. The lo present application is specifically directed to frame locking the hybrid signal to the NTSC signal so that the "visual effect" of the data in the hybrid signal vertical interval is minimized in the NTSC video display. It will, of course, be clear that the many aspects of the inventive system may have benefits that are independent of other aspects of the system, and that the use of one or more of the aspects in combination with each other produces even greater benefits.